An apparatus of this kind is described in Journal Phys. E. Sci. Instruments, Vol. 16, 1983, pp. 313-324. In apparatus of this kind electrons to be detected are preferably generated in a space in which a comparatively strong magnetic field prevails, which electrons are conducted further to a selection space. The electrons then spiral along paths parallel to the optical axis. In the selection space the electrons are directed towards a detector, for example by means of measuring fields to be applied therein. Discrimination as regards kinetic energy of the electrons can be performed, for example by measuring their delay time, by measuring their transmission through a suppression grid, or by measuring their deflection by means of a crossed magnetic-electrostatic field. It would be attractive to perform these measurements in an electron microscope; however, that would have the drawbacks that these measurements are not suitable, for example for Auger electron energies, that the energy resolution is limited, and that the required space with the fields to be generated therein exert a disturbing influence on an irradiating primary electron beam in the microscope.
In general it may be stated that, for example a transmission electron microscope must be capable of focussing the electron beam on a specimen surface to a minimum diameter which is limited only by the optical properties of an objective lens and by the electron wavelength, that the beam position must be stable, and that the beam must be displacable across the specimen surface by means of a beam deflection device. For an Auger electron spectrometer a high electron transmission and a high energy resolution are required and the electron energy to be detected must be adjustable without affecting the position of the irradiating primary electron beam.